Method for Storing Target Comprising Rare Earth Metal or Oxide Thereof

ABSTRACT

Provided is a method for storing a target comprising a rare earth metal or oxide thereof, wherein oxide of the same rare earth metal as the material of the rare earth metal or its oxide target to be stored is introduced as a desiccant into a container or a film-type seal for storing the target, and the target is stored by sealing the storage container or the film-type seal. This invention aims to provide technology for enabling the long-term storage of a target by devising the method for storing a target comprising a rare earth metal or oxide thereof, and thereby inhibiting the pulverization of the target caused by the oxidation and hydroxylation of such target due to the ingress of air.

TECHNICAL FIELD

The present invention relates to a method for storing a targetcomprising a rare earth metal or oxide thereof that is easily pulverizeddue to oxidation or hydroxylation.

BACKGROUND ART

Rare earth metals are contained in the earth's crust as a mixedcomposite oxide. Rare-earth elements are so called because they areseparated from relatively rare minerals, but they are not that rare inlight of the overall earth's crust. In recent years, rare earth metalsare attracting attention as an electronic material, and research anddevelopment for using rare earth metals are being promoted.

Among the rare earth metals, lanthanum (La) is attracting particularattention. To briefly introduce lanthanum, lanthanum is a white metalhaving an atomic number of 57 and an atomic weight of 138.9, andcomprises a double hexagonal close-packed structure at normaltemperature. Lanthanum has a melting point of 921° C., boiling point of3500° C., and density of 6.15 g/cm³, its surface is oxidized in theatmosphere, and it gradually melts in water.

Lanthanum is soluble in hot water and acid. Although it is not ductile,it is slightly malleable. Lanthanum's resistivity is 5.70×10⁻⁶ Ωcm, andit becomes oxide (La₂O₃) when burned at 445° C. or higher (refer toDictionary of Physics and Chemistry). With rare earth elements, it isgenerally said that compounds with the oxidation number 3 are stable,and lanthanum is also trivalent.

Lanthanum is a metal that is attracting attention as an electronicmaterial such as a metal gate material or a high-dielectric constant(High-k) material. Rare earth elements other than lanthanum also haveattributes similar to lanthanum.

A rare earth metal such as lanthanum is a material in which highpurification is difficult to achieve since it is easily oxidized duringthe refining process. In addition, if a rare earth metal such aslanthanum is left in the atmosphere, there is a problem in that thehandling thereof is difficult since it will become oxidized andtarnished in a short time.

In recent years, thinning of a gate insulator film is being demanded inthe next-generation MOSFET, but with the SiO₂ that has beenconventionally used as the gate insulator film, the leak currentincreases due to the tunnel effect, and normal operation is becomingdifficult.

Thus, as a substitute for the SiO₂ described above, HfO₂, ZrO₂, Al₂O₃and La₂O₃ with high dielectric constant, high thermal stability, andhigh energy barrier against the holes and electrons in the silicon havebeen proposed. in particular, among the foregoing materials, La₂O₃ isvalued highly, and a research on its electrical properties and for itsuse as a gate insulator film in the next-generation MOSFET has beenreported (refer to Non Patent Document 1). Nevertheless, with Non PatentDocument 1, the subject of research is a La₂O₃ film, and it does notmake any reference to the properties and behavior of La metal.

It could be said that rare earth metals such as lanthanum and theiroxides are still in the research phase. When studying the properties ofsuch rare earth metals and their oxides, if a rare earth metal or oxidethereof itself exists as a sputtering target material, it is possible toform a thin film of such rare earth metal or oxide thereof on asubstrate. It will be easy to study the behavior at the interface withthe silicon substrate, and the properties of a high-dielectric gateinsulator film or the like after forming a rare earth metal compound.There is also a significant advantage in that the freedom of the targetas a product will increase.

Nevertheless, even if a lanthanum sputtering target is prepared, asdescribed above, it becomes oxidized in a short time in the atmosphere.Generally speaking, a stable oxide layer is formed on a metal targetsurface, but since it is extremely thin, it peels off during the initialstage of sputtering and does not affect the sputtering characteristicssignificantly. However, with a lanthanum sputtering target, the oxidelayer becomes thick, and deterioration of the electrical conductivitywill occur and thereby cause defective sputtering.

In addition, if the lanthanum sputtering target is left in theatmosphere for a long time, it reacts with the moisture in the air tobecome covered with white hydroxide powder and ultimately becomepulverized, and it may even cause a problem of not allowing normalsputtering to be performed. Thus, after the target is prepared, it isnecessary to take measures for preventing oxidation and hydroxylationsuch as by immediately vacuum-packing or coating the target with fatsand oils.

As a method for storing rare earth metals, the standard method is tostore the rare earth metals in mineral oil in order to prevent such rareearth metals from being exposed to the air. However, when using a rareearth metal for a sputtering target, it is necessary to clean the targetbefore its use in order to remove the mineral oil. However, there is aproblem in that the cleaning of the target itself is difficult due toits reactivity with oxygen, moisture and carbon dioxide.

Accordingly, under normal circumstances, it is necessary to store andpackage the target based on vacuum packing. However, even in a statewhere the target is vacuum packed, since pulverization caused byoxidation or hydroxylation will progress even with a small amount ofmoisture that penetrates the used film, it was difficult to store thesputtering target in enabled condition for a long time.

When reviewing the conventional background art, there are the followingmethods; namely, a method of covering the hollow-cathode-type sputteringtarget with a resin bag (refer to Patent Document 1), a method ofcoating the target with a plastic protective film (refer to PatentDocument 2), a method of packaging the target using a film having asurface that does not release particles (refer to Patent Document 3), amethod of preparing a target storage container with a transparent lid ofacrylic resin and screwing shut the storage container (refer to PatentDocument 4), and a method of encapsulating the sputtering target in abag (refer to Patent Document 5). Nevertheless, since the foregoingDocuments disclose that the target is encapsulated with a resin lid or aresin film, they are insufficient as a method for storing a targetcomprising a rare earth metal or oxide thereof.

[Non Patent Document 1] Written by Eisuke Tokumitsu and two others,“Research on Oxide Material for High-k Gate Insulator Film” TheInstitute of Electrical Engineers of Japan, Research Paper of ElectronicMaterials, Vol. 6-13, Pages 37 to 41, Published on Sep. 21, 2001

[Patent Document 1] International Publication No. W02005/037649

[Patent Document 2] Japanese Laid-Open Patent Publication No.2002-212718 [Patent Document 3] Japanese Laid-Open Patent PublicationNo. 2001-240959

[Patent Document 4] Japanese Laid-Open Patent Publication No. H8-246135[Patent Document 5] Japanese Laid-Open Patent Publication No. H4-231461

DISCLOSURE OF THE INVENTION Problems which the Invention Intends toSolve

An object of this invention is to provide technology for allowing thelong-term storage of a sputtering target in enabled condition bydevising the method for storing a target comprising a rare earth metalor oxide thereof, and thereby inhibiting the pulverization of the targetcaused by the oxidation and hydroxylation of such target due to residualair or ingress of air.

Means for Solving the Problems

The present invention provides:

-   -   1) A method for storing a sputtering target comprising a rare        earth metal or oxide thereof, wherein oxide of the same rare        earth metal as the material of the target comprising the rare        earth metal or oxide thereof to be stored is introduced as a        desiccant into a container or a film-type seal for storing the        target, and the sputtering target is stored by sealing the        container or the film-type seal; and    -   2) A method for storing a target comprising a rare earth metal        or oxide thereof, wherein rare earth metal oxide having greater        hygroscopic property than the material of the target comprising        the rare earth metal or oxide thereof to be stored is introduced        as a desiccant into a container or a film-type seal for storing        the target, and the target is stored by sealing the container or        the film-type seal.

The present invention additionally provides:

-   -   3) The method for storing a target comprising a rare earth metal        or oxide thereof according to 2) above, wherein, if the target        comprises two or more types of rare earth metals or their        oxides, an oxide of the rare earth metal with the greatest        hygroscopic property is used as a desiccant;    -   4) The method for storing a target comprising a rare earth metal        or oxide thereof according to any one of 1) to 3) above, wherein        the encapsulating and storing method for the target is based on        vacuum sealing; and    -   5) The method for storing a target comprising a rare earth metal        or oxide thereof according to any one of 1) to 4), wherein the        encapsulating and storing means for the target is a vacuum        sealing with a flexible film.

The present invention further provides:

-   -   6) The method for storing a target comprising a rare earth metal        or oxide thereof according to any one of 1) to 5) above, wherein        the encapsulating and storing method for the target is performed        by filling inert gas having a dew point of −80° C. or less to        encapsulate the target;    -   7) The method for storing a target comprising a rare earth metal        or oxide thereof according to any one of 1) to 6) above, wherein        the rare earth metal oxide to be used as a desiccant is mounted        on or filled in a space created when the target is encapsulated;    -   8) The method for storing a target comprising a rare earth metal        or oxide thereof according to any one of 1) to 7) above, wherein        the rare earth metal configuring the target is La or contains        La;    -   9) The method for storing a target comprising a rare earth metal        or oxide thereof according to any one of 1) to 8) above, wherein        the rare earth metal oxide to be used as a desiccant is La        oxide; and    -   10) The method for storing a target comprising a rare earth        metal or oxide thereof according to any one of 1) to 9) above,        wherein the amount of moisture penetration through the flexible        film to be used in encapsulating and storing the target or the        amount of moisture ingress into the container from the outside        is 0.1 g/m²·24 h or less.

EFFECT OF THE INVENTION

When encapsulating and storing a conventional target comprising a rareearth metal or oxide thereof in a hermetic container or with a plasticfilm, if it goes unattended for a long time, it will react with oxygenand moisture and become covered with white hydroxide powder, and thereis a problem in that normal sputtering cannot be performed.Nevertheless, the target that is stored in a storage container or afilm-type seal according to the present invention yields a significanteffect of being able to avoid the foregoing problems.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 A diagram showing an example where the surface and side face ofthe La target are thinly coated with La oxide powder and vacuum packingis performed thereto.

FIG. 2 A diagram showing an example where La oxide powder is placed in aspace due to the unevenness (difference in level) between the La targetand BP (backing plate) and vacuum packing is performed thereto.

FIG. 3 A diagram showing an example where a La target is placed in ametallic container, La oxide powder is filled around the periphery ofthe La target, ambient air is once replaced with argon having a dewpoint of −80° C. or less, and vacuum sealing is subsequently performedthereto.

FIG. 4 A diagram showing an example where a metal alloy targetcomprising La and Er is placed in a metallic container, a sintered Laoxide block is placed at the uneven part between the target and BP, airin the container is replaced with argon gas, and vacuuming issubsequently performed thereto.

FIG. 5 A diagram showing an example where the La target is subject tovacuum sealing with a film.

FIG. 6 A diagram showing an example where the La target is subject tovacuum sealing with a film, and silica gel is placed as a desiccant.

FIG. 7 A diagram showing an example where the La₂O₃ target is subject tovacuum sealing with a film.

BEST MODE FOR CARRYING OUT THE INVENTION

Rare earths, in particular lanthanum and lanthanum oxide, are known tohave extremely strong hygroscopic properties (reactivity with moisture).Thus, the challenge to date was how to prevent lanthanum and lanthanumoxide from absorbing moisture as much as possible, and how to storelanthanum and lanthanum oxide in an environment with minimal moisture.

Nevertheless, as a result of taking advantage of the fact that lanthanumoxide has a higher hygroscopic property than lanthanum, the oxidation orhydroxylation of the lanthanum target body can be prevented by coating,mounting or placing lanthanum oxide (powder or sintered compact of aplate or block) on the target surface upon storing the lanthanum target.

Moreover, when storing a lanthanum oxide target, moisture can beabsorbed and eliminated more effectively by enclosing the powdered orgranular lanthanum oxide of larger surface area and, therefore,degradation of the target caused by hydroxylation can be prevented.

Specifically, the method for storing a target comprising a rare earthmetal or oxide thereof according to the present invention is tointroduce, as a desiccant, oxide of the same rare earth metal as thematerial of the rare earth metal or its oxide target to be stored into astorage container or a film-type seal.

Here, even if lanthanum oxide reacts with moisture to be hydroxylatedand pulverized and it adheres to the target surface, since it is acompound of the identical metal in a powdered form, the foregoing powderwill not cause any contamination since it can be eliminated easily. Thisis a significant advantage in comparison to the case of using adesiccant made of other metals.

Moreover, in cases where the contamination by other rare earth metalswill not cause a problem: it is possible to introduce oxide of rareearth metal having a greater hygroscopic property than the material ofthe rare earth metal or its oxide target to be stored, as a desiccant,into a storage container or a film-type seal for the target, and storethe target by sealing the storage container or the film-type seal.

If the target is made of two or more types of rare earth metals or theiroxides, an oxide of the rare earth metal with the greatest hygroscopicproperty can be used as a desiccant.

As an encapsulating and storing method for target, it is preferable toprevent the ingress of outside air as much as possible. Vacuum sealingcan be performed as one such method. Moreover, when vacuum sealing andstoring the target, it is preferable to once replace the inside of thecontainer or the film-type seal with inert gas having a dew point of−80° C. or less and thereafter perform vacuum sealing. As means forencapsulating and storing the target, flexible film may be used in theform of a hermetic bag and subject to vacuum sealing.

Although the case of performing vacuum sealing was explained above,inert gas having a dew point of −80° C. or less may be filled and sealedas the method for storing a target. All of the foregoing methods preventthe ingress of outside air.

As described above, contact with outside air is blocked and the ingressof moisture in the outside air is inhibited as much as possible.Nevertheless, even if there is slight ingress of outside air,hydroxylation of the target body can be inhibited by mounting or fillingthe foregoing rare earth oxide to be used as the desiccant in the spacecreated when the target is encapsulated.

A target is generally bonded to a backing plate. For instance, whenusing a flexible film in the form of a hermetic bag and performingvacuum sealing thereto, unevenness will inevitably arise between thetarget and the backing plate, and a space is likely to be formed.Outside air is easily accumulated in such space. Pulverization of thetarget tends to advance from such space. Thus, it is desirable to fillthe rare earth oxide as the desiccant at such uneven part or in suchspace.

In this respect, it should be easy to understand that the rare earthoxide to be used as the desiccant is preferably in the form of powder orgranule of large surface area. Nevertheless, it is effective to simplyplace a small piece of rare earth oxide at a location where outside airis easily accumulated.

Moreover, although it is most effective to place the rare earth oxideand the target to be in direct contact, adhesion of powder to the targetsurface may cause the generation of particles during sputtering. In theforegoing case, a sufficient effect can be yielded even by enclosing therare earth oxide packed with a permeable film as with a generaldesiccant.

The method for storing a target according to the present invention isparticularly effective for a lanthanum target or a target containinglanthanum as the rare earth metal configuring the target. Moreover, theforegoing rare earth oxide to be used as the desiccant is lanthanumoxide. This is ironic, but lanthanum oxide that is most easilyhydroxylated has the greatest inhibitory effect on hydroxylation of atarget comprising a rare earth metal or oxide thereof in the method forstoring a target comprising a rare earth metal or oxide thereof.

It is also important, as the method for storing a target comprising arare earth metal or oxide thereof, to keep the amount of moisturepenetration through the flexible film to be used in encapsulating andstoring the target or the amount of moisture ingress into the storagecontainer from the outside to be 0.1 g/m²·24 h or less and to preventthe ingress of moisture as much as possible.

Table 1 shows the favorable examples of the flexible film to be used inencapsulating and storing the target, as well as the non-favorableexamples.

In Table 1, the examples having properties of GX Barrier (product name)or higher are effective. As shown in Table 1, GX Barrier (product name)and a bag containing Al foil are favorable. Table 1 shows representativeexamples, and it goes without saying that other flexible films may beused so as long as they satisfy the foregoing conditions.

TABLE 1 Permeability Water vapor Oxygen (g/m² · 24 h) (cm³/m² · 24 h ·atm) Eval 4 3 NM Barrier 0.32 0.24 GX Barrier <0.02 0.063 Bag containingAl Foil <0.02 0.047

Examples of implementing the present invention are now explained.

Incidentally, these Examples are merely illustrative, and the presentinvention shall in no way be limited thereby. In other words, variousmodifications and other embodiments based on the technical spiritclaimed in the claims shall be covered by the present invention as amatter of course.

Example 1

This is an example where the surface and side face of a La target arethinly coated with La oxide powder and vacuum packing is performedthereto. A specific example is shown in FIG. 1.

As shown in FIG. 1, as a result of a lanthanum oxide layer existingbetween the vacuum-packing film and the La target, the lanthanum oxidepowder absorbs the moisture remaining inside the vacuum package and themoisture that penetrates the film and is fixed as lanthanum hydroxide.Thus, this is effective in preventing the phenomenon where the La targetsurface reacts with moisture to become hydroxide and be pulverized.

Example 2

This is an example where La oxide powder (as with a silica gel bag) isplaced in a space due to the unevenness with BP and vacuum packing isperformed thereto. A specific example is shown in FIG. 2.

As shown in FIG. 2, since La oxide absorbs the moisture in the slightspace remaining in the vacuum package and is fixed as lanthanumhydroxide. Thus, this is effective in preventing the phenomenon wherethe La target reacts with moisture to become of hydroxide and bepulverized.

Example 3

This is an example where a La oxide target is placed in a metalliccontainer, La oxide powder is filled around the periphery of the Latarget, ambient air is once replaced with argon having a dew point or−80° C. or less, and vacuum sealing is subsequently performed thereto. Aspecific example is shown in FIG. 3.

As shown in FIG. 3, lanthanum oxide absorbs the moisture remaining afterthe vacuuming and is fixed as lanthanum hydroxide. Thus, this iseffective in preventing the phenomenon where La reacts with moisture tobecome hydroxide and be pulverized.

Example 4

This is an example where a metal alloy target comprising La and Er isplaced in a metallic container, a sintered La oxide block is placed atthe uneven part between the target and BP, air in the container isreplaced with argon gas having a dew point of −80° C. or less, andvacuuming is subsequently performed thereto.

As shown in FIG. 4, lanthanum oxide absorbs the moisture remaining afterthe vacuuming and is fixed as lanthanum hydroxide. Thus, this iseffective in preventing the phenomenon where La reacts with moisture tobecome hydroxide and be pulverized.

Comparative Example 1

This is an example where the La target was subject to vacuum sealingwith a film. A specific example is shown in FIG. 5. As shown in FIG. 5,the small amount of moisture remaining in the space created between thevacuum-packing film and the target reacted with the La target, and Labecame lanthanum hydroxide. It resulted in pulverization.

Comparative Example 2

This is an example where the La target was subject to vacuum sealingwith a film, and silica gel was placed as a desiccant. A specificexample is shown in FIG. 6.

When silica gel is placed as shown in FIG. 6, pulverization progressedfaster than the case of not placing silica gel. The reason for this isconsidered to be that the moisture adsorbed by the silica gel becamedesorbed and discharged inside the vacuum-packing film, and the reactionof moisture and La was promoted and resulted in lanthanum hydroxide. Inlight of the above, it is evident that silica gel that is used as ageneral desiccant is useless, or even undesirable, in preventing thepulverization phenomenon of the rare earth metal or rare earth metaloxide.

Comparative Example 3

This is an example where the La₂O₃ target was subject to vacuum sealingwith a film. A specific example is shown in FIG. 7. As shown in FIG. 7,since La oxide (La₂O₃) absorbs the moisture in the slight spaceremaining in the vacuum package and is fixed as lanthanum hydroxide, theLa₂O₃ target reacts with moisture, and La₂O₃ becomes hydroxide. Itresults in pulverization.

As evident from the foregoing Examples and Comparative

Examples, upon storing a target comprising a rare earth metal or oxidethereof, it is evident that the introduction of oxide of the same rareearth metal as the material of the target comprising the rare earthmetal or oxide thereof to be stored as a desiccant into a hermeticcontainer or seal is extremely effective. It is thereby possible toeffectively inhibit the pulverization of the target caused by theoxidation and hydroxylation of such target due to residual air oringress of air.

INDUSTRIAL APPLICABILITY

Conventionally, if a rare earth metal or a rare earth metal oxidesputtering target is left in the atmosphere for a long time, it reactswith the moisture in the air and becomes covered with white hydroxidepowder, and there is a problem in that normal sputtering cannot beperformed. Nevertheless, the method for storing a target comprising arare earth metal or oxide thereof according to the present inventionenables to avoid the foregoing problems.

The method for storing a target comprising a rare earth metal or oxidethereof according to the present invention is to introduce, as adesiccant, oxide of the same rare earth metal as the material of thetarget comprising the rare earth metal or oxide thereof to be storedinto a storage container or a film-type seal. It is thereby possible toeffectively inhibit the condition where the target reacts with themoisture in the atmosphere and becomes covered with white hydroxidepowder.

Accordingly, targets can be stably supplied as an electronic materialsuch as a metal gate material or a high-dielectric constant (High-k)material, and the present invention is extremely useful industrially.

1. A method for storing a target comprising a rare earth metal or oxidethereof, wherein oxide of the same rare earth metal as the material ofthe target comprising the rare earth metal or oxide thereof to be storedis introduced as a desiccant into a container or a film-type seal forstoring the target, and the target is stored by sealing the container orthe film-type seal.
 2. A method for storing a target comprising a rareearth metal or oxide thereof, wherein rare earth metal oxide havinggreater hygroscopic property than the material of the target comprisingthe rare earth metal or oxide thereof to be stored is introduced as adesiccant into a container or a film-type seal for storing the target,and the target is stored by sealing the container or the film-type seal.3. The method for storing a target comprising a rare earth metal oroxide thereof according to claim 2, wherein the target comprises two ormore types of rare earth metals or their oxides, and wherein an oxide ofthe rare earth metal with the greatest hygroscopic property is used asthe desiccant.
 4. The method for storing a target comprising a rareearth metal or oxide thereof according to claim 3, wherein the step ofsealing is vacuum sealing.
 5. The method for storing a target comprisinga rare earth metal or oxide thereof according to claim 3, wherein thestep of sealing is vacuum sealing with a flexible film.
 6. The methodfor storing a target comprising a rare earth metal or oxide thereofaccording to claim 3, further comprising the step of filling inert gashaving a dew point of −80° C. or less into the container or film-typeseal and encapsulating the target.
 7. The method for storing a targetcomprising a rare earth metal or oxide thereof according to claim 6,wherein the rare earth metal oxide to be used as the desiccant ismounted on or filled in a space created when the target is encapsulated.8. The method for storing a target comprising a rare earth metal oroxide thereof according to claim 2, wherein the rare earth metalconfiguring the target is La or contains La.
 9. The method for storing atarget comprising a rare earth metal or oxide thereof according to claim8, wherein the rare earth metal oxide to be used as the desiccant is Laoxide.
 10. The method for storing a target comprising a rare earth metalor oxide thereof according to claim 9, wherein the amount of moisturepenetration through the flexible film to be used in encapsulating andstoring the target or the amount of moisture ingress into the containerfrom the outside is 0.1 g/m²·24 h or less.
 11. The method according toclaim 2, further comprising the step of filling inert gas having a dewpoint of −80° C. or less into the container or film-type seal andencapsulating the target.
 12. The method according to claim 2, whereinthe oxide to be used as the desiccant is mounted on or filled in a spacecreated when the target is encapsulated by the container or film-typeseal.
 13. The method according to claim 2, wherein an amount of moisturepenetration through the flexible film to be used in encapsulating andstoring the target or the amount of moisture ingress into the containerfrom the outside is 0.1 g/m²·24 h or less.
 14. The method according toclaim 1, wherein the step of sealing is vacuum sealing.
 15. The methodaccording to claim 1, wherein the step of sealing is vacuum sealing witha flexible film.
 16. The method according to claim 1, further comprisingthe step of filling inert gas having a dew point of −80° C. or less intothe container or film-type seal and encapsulating the target.
 17. Themethod according to claim 1, wherein the oxide to be used as thedesiccant is mounted on or filled in a space created when the target isencapsulated by the container or film-type seal.
 18. The methodaccording to claim 1, wherein the rare earth metal configuring thetarget is La or contains La.
 19. The method according to claim 19,wherein the oxide to be used as the desiccant is La oxide.
 20. Themethod according to claim 1, wherein an amount of moisture penetrationthrough the flexible film to be used in encapsulating and storing thetarget or the amount of moisture ingress into the container from theoutside is 0.1 g/m²·24 h or less.